Systems for optical inspection of strands, such as thread, optical fiber and polymer filaments are known. One such sensing head utilizing three noncolinear light beams is marketed by Takikawa Engineering of Tokyo, Japan. In this system the three light beams are detected after being incident on the strand being monitored and each signal is monitored to produce an alarm signal if the monitored signal exceeds a threshold indicating the presence of an unacceptably large fault. For demanding, high tolerance applications, such as sutures to be used in heart operations, systems of greater sensitivity are desired.